1. Field of the Invention
This invention relates generally to a spin torque transfer (STT) magnetic random access memory (STT-MRAM) cell formed in a magnetic tunneling junction (MTJ) configuration and patterned in a C-shape. In particular, it relates to the use of an additional word line to provide a pulsed magnetic field that assists in switching the magnetization of such a cell.
2. Description of the Related Art
The conventional magnetic tunneling junction (MTJ) device is a form of ultra-high magnetoresistive (MR) device in which the relative orientation of the magnetic moments of parallel, vertically separated magnetized layers, controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those layers.
Referring to FIG. 1 there is shown a highly schematic illustration of a prior art MTJ cell that, for the purposes of the following descriptions, can be either a standard MTJ cell or, as will be discussed below, a spin torque transfer (STT) cell. Although the physics of the two types of cell operation are quite different, the cell structures have striking similarities. The figure includes an overhead view of the cell, showing it to have an elliptical horizontal cross-sectional shape. Beneath the elliptical overhead view, the cell is shown in vertical cross-section, revealing three “active” layers, all of which play a role in providing its physical properties.
The three active layers include: a fixed layer (110) formed of magnetic material, a tunnel barrier layer (120) formed of dielectric material and a free layer (130), also formed of magnetic material. Arrows (132) represent the two possible directions of the magnetic moments of the free layer and arrow (133) represents the single magnetization direction of the fixed layer. In the case of the standard MTJ cell, the magnetic moment of the free layer is made to move from one direction to the other under the action of the external magnetic fields of nearby current carrying wires.
In the case of the STT type of MTJ cell, to be described in greater detail below, the free layer magnetic moment is made to move by the action of electron torques produced by currents passing through the cell rather than by the magnetic fields of current carrying wires that are external to the cell. In either case, the magnetization of the fixed (or pinned) layer is held in place by an interaction with a neighboring layer (the pinning layer) that is not specifically shown here. Because the magnetization of the free layer must be relatively free to move under whichever of the mechanisms applied, there must be a mechanism to stabilize the direction of its magnetic moment after it has been moved to a desired orientation and the moving force has been removed. This is particularly true in the case of increasingly small cells, where random thermal destabilizing effects can be comparable to the intentional effects used to switch the moment directions.
One way of stabilizing such an MTJ cell is to provide it with a shape anisotropy by patterning it with, for example, an elliptical cross-section as shown in FIG. 1. This cell shape tends to stabilize the magnetization along either direction of the longer elliptical axis, called the easy axis, so that a certain minimum field or torque is required to change the magnetization from one direction to the other.
Referring to FIG. 2, there is shown a more detailed, yet still schematic illustration of a vertical cross-sectional view of two adjacent MTJ cells (100) formed in a STT configuration. Each cell is positioned beneath a common current carrying line (200), the bit line. Going vertically downward in either cell, there is first shown a capping layer (140) contacting the bit line and protecting the cell, beneath the capping layer there is shown a magnetically free layer (130) whose magnetic moment is free to move, beneath the free layer there is shown a dielectric tunneling barrier layer (120), beneath the tunneling barrier layer there is shown a pinned layer (110) of fixed magnetic moment, beneath the pinned layer there is shown a seed layer (105), on which is deposited the pinned layer, below the seed layer is shown a bottom electrode (150), which makes electrical contact with the cell, beneath the bottom electrode there is shown a cross-section of a stud (90) that facilitates connection to an accessing transistor (80) and, finally, there is shown a schematic circuit of such an accessing transistor (80), connected to ground, that allows a current between the bit line and ground through either of the MTJ cells, if they are selected.
The switching of the free layer (130) magnetization can be achieved by using the STT mechanism, as described by J. C. Slonczewski, “Current-driven excitation of magnetic multilayers,” J. Magn. Magn. Mater., vol. 159, pp. L1-L7, 1996, J. Sun, “Spin-current interaction with a monodomain magnetic body: A model study,” Phys. Rev. B 62, 570 (2000) and as further disclosed by Sun, (U.S. Pat. No. 6,130,814). In the STT design, the direction of the current through the free layer (130) will determine whether its magnetization is parallel to (P) or anti-parallel to (AP) the magnetization of the pinned layer (110). To change the direction of free layer magnetization (i.e., to “write” on the free layer) from AP to P, the electrons (current) must move from the pinned layer (free layer) to the free (pinned) layer. After passing through the pinned layer, the spin direction of the majority of electrons in the current will have the same direction as the magnetic moment of the pinned layer. This is a result of the torque exerted on the spinning electron by the predominant magnetic moment of the pinned layer. The electrons then pass through the dielectric barrier layer (120), which preserves their spin direction. Since the free layer's magnetic moment is opposite to that of the pinned layer, the majority of the electron's spins will be opposite to that of the free layer and will interact with the magnetization of the free layer near the interface between the free layer and the dielectric barrier layer. Through this interaction, the spin of the transmitted electrons will be transferred to the free layer. When the electron current exceeds a critical value, Ic, there will be sufficient momentum transfer from the current to the free layer to switch the magnetization direction of the free layer from the AP state to the P state. To write from P to AP, on the other hand, the electrons must flow in the opposite direction, from the free layer to the pinned layer. After being transmitted through the free layer, the majority of the electrons will have their spins directed along the magnetization direction (P) of the free layer and the pinned layer as well. They can therefore, be transported through the pinned layer with very little scattering. The minority of the electrons, with their spins opposite to the magnetization of the free and pinned layers, will be reflected back to the free layer by the pinned layer and will transfer their polarizations to the free layer at that interface between the barrier layer and the free layer. Once the number of minority electrons in the current is sufficient, the magnetization of the free layer can be switched to the AP state.
For practical applications to high density memory circuits, the critical current should be low. The current is provided by the transistor connected to the MTJ element and the transistor's size determines the density of the memory. Also, an MTJ element with a dielectric spacer (tunneling barrier) layer such as MgO is the preferred choice due to the fact that it can provide a magnetoresistive ratio, DR/R, up to 600%, which is critical for memory read signal and speed. To avoid dielectric breakdown of the MTJ, the voltage, Vc, across the dielectric layer at the critical current, Ic, which is given by: Vc=RIc, has to be lower than the breakdown voltage of the barrier layer. This means that the low critical current (or current density, Jc) is essential for the MRAM product.
Numerous efforts have been made trying to reduce the critical current or current density. According to J. Sun, cited above, Jc is proportional to (α/P) Mst (Heff−2πMs), where α is the Gilbert damping constant, P is the spin transfer efficiency, Ms is the magnetization of the free layer, t is the thickness of the free layer, Heff is the effective magnetic field, including the external magnetic field, the shape anisotropy field, the exchange field between the free and pinned layers and the dipole field from the pinned layer.
One design proposed to reduce Jc is disclosed by Saito, in U.S. Pat. Nos. 7,239,541 B2, 7,248,497 B2 and 7,511,991 B2, which is to add an additional word line, insulated from the existing MTJ cells, by which a magnetic field is generated along the easy axis of the cells, opposite to the direction of free layer magnetization. This word line field will reduce the value of Heff and, therefore, will reduce Jc. However, this design causes the traditional “half-select” problem, in which some of the cells not intended to be programmed, but beneath the additional word line with their free layer magnetizations in an opposite direction to the word line field, can be disturbed into their opposite directions. To avoid this problem, the value of the shape anisotropy field must be increased, but this, again, raises the value of Jc.
One way to solve the half-select problem, not in the context of the STT scheme however, is to induce the C-switching mode (“Switching field variation in patterned submicron magnetic film elements,” Youfeng Zheng, Jian-Gang Zau, J. Appl. Phys., 81(8), 15, p5471, 1997) by patterning the MTJ cells into a C-shaped horizontal cross-section, as is disclosed by Katti et al. (U.S. Pat. No. 6,798,690 B1), Ounadjela et al. (U.S. Pat. No. 6,798,691 B1), Inokuchi et al. (U.S. Pat. No. 7,394,684), Ikegawa et al. (U.S. Pat. No. 7,518,906), Kishi et al. (U.S. Pat. No. 7,599,156) and Min et al. (US Publ. Pat. Appl. 20080253178 A1), which latter published application (Min et al.) is assigned to the same assignee as the present application and whose contents are fully incorporated herein by reference. This C-switching mode approach confines the free layer field into this C-shape, whose non-coherent C-mode switching, as fully described in the references cited above, requires a much higher switching field and thereby provides immunity to disturbance by the additional word line field.